1. Field of the Invention
The present invention relates to a method for storing a polyester film for use as a magnetic recording medium, and a method for producing a magnetic recording medium by forming a thin ferromagnetic metallic film on the surface of the polyester film stored according to the storing method.
The polyester film of this invention is preferably used as a base film of a video cassette tape or data storage tape, etc. used for recording digital data.
2. Description of the Prior Art
The residential and commercial use digital video tape (magnetic recording medium) which began to be practically used in 1995 is produced by forming a thin ferromagnetic metallic film of cobalt (Co) on a 6 to 7 .mu.m thick base film by vacuum evaporation, and furthermore coating the surface with a diamond-like carbon film. The digital video tape is used for a mini cassette of a digital video (DV) apparatus. A camera-integrated video cassette recorder mounted with this cassette allows one-hour picture recording in its basic specification (standard definition (SD)).
The digital video cassette (DVC) was the first home use digital video cassette in the world, and has the following advantages, being highly evaluated on the market: it allows an enormous volume of information to be recorded in its small body; will resist degradation in image and sound quality even after years, since its signals are little degraded; its allows high quality image and sound to be enjoyed since it is not susceptible to noise interference, and is little degraded in image even after repeated dubbing.
The films which are used as the base films of magnetic recording media include the following:
(A) A polyester film comprising a polyester film, a polymer blend deposited at least on one side of the film, and a discontinuous film which is mainly composed of fine particles with a particle size of 50 to 500 .ANG. and formed on the surface of the polymer blend and which contains a water soluble polyester copolymer, to form fine projections on the discontinuous film by the fine particles. This polyester film is disclosed in Japanese Patent Publication (Kokoku) No. Sho 63-57238 (U.S. Pat. No. 4,568,600); or PA1 (B) A laminated film in which a layer is made of a thermoplastic resin (e.g., a polyester) and a layer is made of a thermoplastic resin (e.g., a polyester) containing fine particles, as disclosed in Japanese Patent Publication (Kokoku) Hei 1-26338 (U.S. Pat. No. 4,550,049).
However, any of these polyester base films is likely to precipitate an oligomer in the polyester film onto a surface while it is stored after production, until the time when it is used for producing a magnetic recording medium.
The tape (magnetic recording medium) prepared by using a base film with an oligomer precipitated on a film surface is confronted with a problem that undesirable block-shaped portions devoid of image appear on a screen when a recorded image is reproduced. These are called "drop outs".